In recent years, it has been recognized that both chemical synthesis and chemical degradation are preferably performed using technologies that are more sustainable, less hazardous, less polluting and with less byproduct waste. Among other technologies, photocatalysts have been recognized as desirable.
Over the years many have attempted and considerable effort has been applied to develop photocatalysts which can act as self-cleaning coatings, anti-microbial coatings and surfaces degrading organic contaminants, particularly those not readily biodegradable. Titanium dioxide (TiO2) has been of particular interest due to its low cost, almost no toxicity, chemical stability (both to light and the environment) and high photoactivity.
TiO2 is a semiconductor with a number of properties pertinent to photocatalysis such as transparency to visible light, high refractive index and low absorption coefficient. TiO2 has been used in a wide range of applications including ultraviolet filters for optics and packing materials, environmental remediation, papermaking, ceramics, solar cells, electrochromic displays, anodes for ion batteries, self-cleaning coatings and paints and humidity as well as gas sensors.
A large number of prior state of the art references have mentioned using titanium dioxide as a photocatalyst for an assortment of chemical reactions and antimicrobial activity. Many attempts have been made to modify the photocatalytic activity by doping the titanium dioxide with a number of different compounds and using a number of different techniques. Several different nitrogen-containing compounds have been tried using a variety of different doping reactions. However, these attempts have limited stability and efficiency or were active only or primarily under UV light.
Of particular interest has been the rutile and anatase crystalline phases of TiO2. TiO2 has been used extensively under ultraviolet irradiation (UV) due to its large band gap of 3.2 eV. TiO2 exhibits high reactivity and chemical stability under ultraviolet (UV) irradiation at wavelength 387 nm, whose energy exceeds the band gap of 3.0 eV and 3.2 eV for rutile and anatase crystalline phase, respectively. (Asahi et al, Science 293, 269 (2001) and Jagadale et al, J. Phys. Chem. C, 2008, 112, 14595.)
Due to the size of its band gap, pristine TiO2 is active only under UV irradiation, which comprises less than 5% of solar light energy. While functional under UV irradiation, photocatalysis generally does not occur in indoor areas under conventional artificial light or even ambient daylight as UV is not present.
A number of attempts have been made to modify TiO2 to enhance its activity by doping the crystalline structure with a variety of compounds including those with nitrogen, carbon or sulfur atoms. Some attempts have been made to obtain visible light activation of the photocatalysts by the red shift of the adsorption spectrum. Nitrogen atoms have attracted the most attention because its p state contributes to the band gap narrowing by mixing with the oxygen 2p states. (Asahi et al, Science 293: 269 (2001).) Nitrogen doping has been performed by using nitrogen gas, ammonium chloride, ammonia gas and a number of nitrogen containing organic compounds.
Other compounds including noble metals and non-metal species deposited on TiO2 may show different effects on the photocatalytic activity of TiO2 under solar and artificial visible light irradiation. (Kisch et al, Angew. Chem., Int. Ed., 37: 3034 (1998). According to Sung-Suh et al, J. Photochem. Photobiol. A—Chem. 163: 37 (2004)) There are several mechanisms that are responsible for such effects: i) dopants enhance the electron-hole separation by acting as electron traps, ii) they extend the light absorption into the visible range and iii) e.g. noble metals modify the surface properties of the photocatalyst. Metal dopants affect the surface properties by generating a Schottky barrier of the metal in contact with TiO2 surface, which acts as an electron trap and inhibits e−-h+ recombination (Zhou et al, Ind. Eng. Chem. Res., 45: 3503 (2006)). Silver is a metal that is suitable for numerous industrial applications. It has been reported that silver deposited onto TiO2 significantly shortens illumination period and increases the efficiency of the catalyst (Soökmen et al, J. Photochem. Photobiol. A—Chem., 147: 77 (2002)).
A number of different metals, especially transition metals, when used as dopants, cause the titanium dioxide to increase adsorption of visible light. Unfortunately, many of these result in a reduction of photocatalytic activity in the UV range and also are not sufficiently stable to prevent rapid loss of photocatalytic activity of the catalyst.
Nitrogen-doping has been effective in decreasing the band gap of TiO2 through mixing of N 2p and O 2p states due to the electronic transitions from the dopant 2p or 3p orbitals to Ti 3d orbitals (Fu et al, J. Phys. Chem. B, 110, 3061. (2006)). Such doping is also attractive because of comparable atomic size of nitrogen with oxygen, small ionization energy, metastable center formation, and remarkable stability (Jagadale et al., J. Phys. Chem. C, 112, 14595 (2008)).
References mentioning doped titanium dioxide photocatalysts that show any photocatalytic activity under visible light conditions are Sakthivel et al, Angew. Chem. Int. ed. 42: 4908 (2003), Matsushita et al, Journal of the Ceramic Society of Japan, Supplement 112-1, PacRim5 Special Issue, 112[5] S1411 (2004) and Nosaka et al, Science and Technology of Advanced Materials, 6: 143 (2005).
There is a lack of studies utilizing ‘green’ nanoscience principles to fabricate noble metal and non-metal co-doped TiO2 catalysts utilizing renewable sources or various waste materials) Hamal et al, J. Coll. Interf. Sci, 311, 514 (2007)). Also, there is a lack of proven photocatalytic effectiveness using ambient light, particularly in poorly illuminated areas. It was to address the problems indicted above that the present invention was pursued.